The cost of bringing a new drug to the market is extremely high. Typically, a pharmaceutical company will screen hundreds to hundreds of thousands of compounds, in order to choose a single drug for marketing. Initial screening is performed in vitro with the most promising compounds progressing to animal studies. It is on the basis of these animal studies that the best drug(s) is chosen for further development and clinical trials. Since a considerable amount of the cost associated with drug development occurs subsequent to the animal studies, the accuracy of the animal model at predicting a drug's behaviour in humans, is of obvious importance.
During drug discovery and development, animal models are used in an iterative process of characterising the drug candidates. Initial animal studies determine the pharmacokinetics (the kinetics of drug absorption, distribution throughout the body and its eventual elimination from the body). Subsequent animal studies measure pharmacodynamics (mechanisms of drug action, and the relationship between drug concentration and effect). Typically, these studies also look at efficacy (e.g. does the compound block tumour growth, or is the compound effective in combating neurological disorders), short-term toxicity, optimal dosing and scheduling etc. Based on these animal studies, the most promising compound(s) is further developed. This stage involves continued animal studies (e.g. longer term toxicity studies, exhaustive metabolic studies, multiple-administration pharmacokinetic studies, expanded efficacy studies often including drug combination studies), chemical development (e.g. production) and pharmaceutical development (e.g. drug formulation and delivery). Compounds that successfully complete all of these stages are then tested in human patients (Phase I-III clinical trials). A successful Phase I trial would demonstrate good tolerability, suitable pharmacokinetics, and in some cases demonstrate the intended pharmacodynamic properties in humans. Such a drug would progress to Phase II and Phase III clinical trials for testing of the optimal dosing regime as well as efficacy in the treatment of disease.
The development of a single drug often requires the testing of many different compounds in mice or other animal species. These animal studies determine the choice of compound for further development and clinical trials. The main reasons for drug failure at the clinical trial stage are inappropriate pharmacokinetics and toxicological effects (often both are related to drug metabolism), and to a lesser extent, lack of efficacy due to failed concept or lack of pharmacodynamic effect. That compounds progress to clinical trials and then fail at this late stage is usually due to the poor predictability of existing animal models.
An incorrect decision, based on data from an animal model that did not accurately reflect drug behaviour in humans, can waste vital resources, many millions of dollars and years of labour. Moreover, the opportunities lost by not pursuing other candidate compounds could potentially cost billions in lost revenue. For example, current mouse models often (and unpredictably) do not accurately reflect human drug pharmacokinetics, metabolism and toxicology. Many drugs show promising results in mice but fail to work effectively in humans. Similarly, other drugs, which failed in mice and consequently were rejected, may have worked well in humans. Thus, the process of selecting candidate drugs for further development and clinical trials is currently based on data obtained from a flawed animal model. The consequences of this include; a) wastage of valuable resources pursuing drugs that will not work in humans; b) lost opportunities by not pursuing drugs which would work in humans and c) exposure of patients to unknown risks in Phase 1 clinical trials.
An improved animal model, which more accurately predicts the behaviour (e.g. distribution, metabolism, efficacy and toxicity) of drugs in the animals of interest (e.g. humans and other mammals including livestock animals and companion animals) would provide enormous benefit to both the pharmaceutical and/or veterinary industries and to the treatment of diseases affecting those animals.